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    SubjectRe: [RFC PATCH] s390: mm: rmap: Transfer storage key to struct page under the page lock
    On Tue, Apr 17, 2012 at 08:52:21PM -0700, Hugh Dickins wrote:
    > > > (You do remind me that I meant years ago to switch swapper_space over
    > > > to the much simpler __set_page_dirty_no_writeback(), which shmem has
    > > > used for ages; but as far as this problem goes, that would probably
    > > > be at best a workaround, rather than the proper fix.)
    > >
    > > It would be a workaround. If in the future we wanted to treat swapper
    > > space more like a normal file inode and writeback dirty pages from
    > > the flusher thread then this bug would just pop its head back up.
    > It's a no-brainer workaround: patch and more explanation below. I
    > can double-fix it if you prefer, but the one-liner appeals more to me.

    Ok, fair enough. While I think swapper space will eventually use the dirty
    tag information that day is not today.

    > > > Hmm, mm/migrate.c.
    > >
    > > Migration moves the page mapping under the tree lock so
    > > __set_page_dirty_nobuffers() I don't think that is it.
    > Yes, I was worried by the places that set page->mapping = NULL in
    > migrate.c (later, not under the tree_lock), but those would not be able
    > to generate this issue at all (ptes already replaced by migration entries).


    > <SNIP>
    > [PATCH] mm: fix s390 BUG by using __set_page_dirty_no_writeback on swap
    > Mel reports a BUG_ON(slot == NULL) in radix_tree_tag_set() on s390 3.0.13:
    > called from __set_page_dirty_nobuffers() when page_remove_rmap() tries to
    > transfer dirty flag from s390 storage key to struct page and radix_tree.
    > That would be because of reclaim's shrink_page_list() calling add_to_swap()
    > on this page at the same time: first PageSwapCache is set (causing
    > page_mapping(page) to appear as &swapper_space), then page->private set,
    > then tree_lock taken, then page inserted into radix_tree - so there's
    > an interval before taking the lock when the radix_tree slot is empty.

    Yes, makes sense.

    > We could fix this by moving __add_to_swap_cache()'s spin_lock_irq up
    > before SetPageSwapCache, with error case ClearPageSwapCache moved up
    > under tree_lock too.

    This can be done if/when swapper_space can make proper use of the dirty
    tag information.

    > But a better fix is just to do what's five years overdue. Ken Chen
    > added __set_page_dirty_no_writeback() (if !PageDirty TestSetPageDirty)
    > for tmpfs to skip all that radix_tree overhead, and swap is just the same:
    > it ignores the radix_tree tag, and does not participate in dirty page
    > accounting, so should be using __set_page_dirty_no_writeback() too.
    > Reported-by: Mel Gorman <>
    > Signed-off-by: Hugh Dickins <>

    Acked-by: Mel Gorman <>

    I've sent a kernel based on this patch to the s390 folk that originally
    reported the bug. Hopefully they'll test and get back to me in a few

    Thanks Hugh.

    Mel Gorman
    SUSE Labs

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